CMS-11876 a check has been introducded:
First of all, we shouldn't use Validate.isTrue in production code. It just throws an IllegalArgExc leaving the UI broken. We need to handle corner cases. Apart from that, next to HippoStdNodeType.NT_FOLDER we als have hippostd:directory which is now broken.
I suggest to remove the Validate.isTrue altogether: it is AbstractDocumentWorkflowPlugin ....what else can it be than the handle and the parent a folder?